1. INTRODUCTION

Dwarf galaxies, by definition of their class, are galaxies of low
mass and size. This directly implies that they have a much
weaker gravitational potential well than typical spiral galaxies and
fill a smaller volume with gas and stars. All dwarf galaxies, even very
low mass systems, show quite complicated star formation histories (e.g.
Mateo 1998).
From the color-magnitude diagrams one derives the presence of some low
level star formation activity over most of the lifetime, sometimes
interrupted by short intervals of strongly enhanced star formation
rate. These results and the presence of blue
compact dwarf galaxies, where the current star formation rate
is so high that these dwarf galaxies appear as isolated giant HII regions,
tells us that bursts of star formation are indeed a natural process in
dwarf galaxies, as predicted by models of stochastic self
propagating star formation
(Gerola et
al. 1980).
Bursts of star
formation, meaning spatially and temporally correlated energy
input from massive stars and supernova explosions inside physically small
systems, lead to a strong response of the gas in the host galaxy.

Analytical and numerical
modeling of the reaction of the gas to the energy input from stellar winds
and supernovae is an active topic since the papers of
Castor et
al. (1975) and
Weaver et
al. (1977).
Recent examples are e.g.
Freyer & Hensler
(2000),
Strickland & Stevens
(1999),
Mac Low & Ferrara
(1999),
and Tomisaka (1998).
For a review of the basic ideas, see
Tenorio-Tagle &
Bodenheimer (1988).
The result of the energy input into the interstellar medium (ISM) is
basically an expanding bubble of hot gas inside the substrate of cool
gas of the host galaxy. The hot bubble is enclosed by a dense cool
shell, which has
ionized gas at the inner boundary layer between the hot gas and the shell.
If the bubble grows to a linear diameter comparable to the neutral
gas scale height of the galaxy, the expansion speeds along the z-axis
(e.g.
Mac Low & McCray
1988)
and the shell expands into the lower halo of
the host galaxy while starting to deform and break due to Rayleigh-Taylor
instabilities (e.g.
Mac Low et
al. 1989).

Since massive stars and supernova explosions are the dominant source of
heavy elements, the newly processed material is located inside the
shells. Whether and to which degree it is moved upward out of a
galaxy and what happens to this gas in the lower halo, is of crucial
importance for the understanding of the chemical evolution of
dwarf galaxies (e.g.
Hensler & Rieschick
1999).
In the work
Mac Low & Ferrara
(1999)
for the first time a set of hydrodynamical simulations
incorporating a relatively detailed model of the
dwarf galaxy potential (including dark matter) was used and the
simulations for a whole dwarf galaxy was run for more that 100 Myr.
Still, the authors had to compromise e.g. by relatively
basic treatment of the cooling processes and ignoring magnetic fields.

The hot gas inside the bubbles is predicted to be in the temperature
range of 105 and 107 K (e.g.
Weaver et
al. 1977),
which implies that
the plasma will radiate in the extreme UV and soft X-ray regime. The
ionized gas at the boundary layer between the hot interior and the cool
shell wall
should be visible in optical and UV emission lines, while the cool shell
itself can be observed in 21cm emission.
Since density of the substrate medium and especially the size of the energy
depositing stellar association (from few O or even B stars to many
thousands of OB stars as e.g. inside giant HII regions like 30 Dor or
NGC 5471 in M101)
span a large parameter space, the size of the bubbles can vary a lot,
from pc to kpc scale
(Chu 1995).
The (more abundant) small bubbles do not break out of the disk of a galaxy.
These bubbles do still structure the interstellar medium and should lead to
ionized filaments, as observed
in the Magellanic Clouds (e.g.
Kennicutt et
al. 1995)
and the Milky Way
(Haffner et
al. 1999).
They also lead to large regions of hot gas observed
in the LMC (e.g.
Chu & Mac Low 1990,
Bomans et al. 1994)
and the Milky Way (e.g.
Snowden et
al. 2000).

The observation of warm and hot gas in dwarf galaxies allows therefore
to study the mechanism shaping the topology and phase structure of the
ISM as well as the processes responsible for the chemical and (at least
partly) the dynamical evolution of the host galaxy. Dwarf galaxies are
supremely suited for this task. They present the most extreme environment
for feedback of massive stars on the interstellar medium
due to their shallow potential wells, their small sizes, and the absence
of complicating other factors like density waves.